Summary Amphicarpy is an unusual trait where two fruit types develop on the same plant: one above and the other belowground. This trait is not found in conventional model species. Therefore, its development and molecular genetics remain under‐studied. Here, we establish the allooctoploid Cardamine chenopodiifolia as an emerging experimental system to study amphicarpy. We characterized C. chenopodiifolia development, focusing on differences in morphology and cell wall histochemistry between above‐ and belowground fruit. We generated a reference transcriptome with PacBio full‐length transcript sequencing and analysed differential gene expression between above‐ and belowground fruit valves. Cardamine chenopodiifolia has two contrasting modes of seed dispersal. The main shoot fails to bolt and initiates floral primordia that grow underground where they self‐pollinate and set seed. By contrast, axillary shoots bolt and develop exploding seed pods aboveground. Morphological differences between aerial explosive fruit and subterranean nonexplosive fruit were reflected in a large number of differentially regulated genes involved in photosynthesis, secondary cell wall formation and defence responses. Tools established in C. chenopodiifolia, such as a reference transcriptome, draft genome assembly and stable plant transformation, pave the way to study amphicarpy and trait evolution via allopolyploidy.